Why Do I Need a Crank Angle Sensor on My E-bike?

A crank angle sensor on an e-bike is a big step up from a traditional cadence sensor. By using an absolute angle sensor on the crank, the motor controller gets one more input parameter for the central algorithm that controls how the motor responds to the rider’s input. An angle sensor shows the precise position of the crankset. By looking at how quickly the angle changes, it is easy to calculate the cadence as well.

There are several advantages of using an absolute angle sensor to detect crank movement for an e-bike which is why this is the preferred method for higher-end e-bikes and cargo bikes. The main advantages are summarized below:

• Faster start response. As EU regulation prohibits the motor from starting before the crank is moving, it is essential to have a fast detection of this. An angle sensor provides really fast detection and can typically see crank movements down to 1 or 2 degrees of angle.
• No magnets. Getting rid of magnets is a big step up in any product just from a sourcing, manufacturing, and lifetime point of view. For a magnet ring to provide the same fast response as an angle sensor, the number of magnets needs to be in the range of 3-400. This is often not practical or economical.
• Easy start from any crank position. A typical problem on 3-wheel cargo bikes is how to get started when the crank is in a vertical position. Even worse if the start is on an incline. At that crank angle, the human anatomy does not allow a lot of force to be put into the pedals, so the value seen by the torque sensor is quite low. A smart motor controller can detect this situation and add some extra motor assistance to get the heavy bike started providing a superior rider experience.
• Angle-dependent torque scaling/dead-zone removal. As the muscles of the legs are not able to provide the same torque at all angles – even if using clips on the pedals – some motor controller algorithms will scale the torque measured by the torque sensor by what torque output is possible at that angle. This can provide a smoother ride where a small torque in the top/bottom position of the crank (dead zone) weighs in just as much as a naturally larger torque measured in the down-stroke part of the crank rotation.

You can read more about the torque “peanut” plot and how the human body works on a bike online (“Peanuts or 8’s – Pedal Action” and “The Wattbike Atom: Sweat the technique” as good examples).